1. Field of the Invention
The present invention relates to a system applied to a network where a connection is identified based on a link identifier according to a protocol such as MPLS (Multiprotocol Label Switching) and so on.
2. Description of Related Art
A connection-oriented communication is one category of communication methods. One of the networks adopting the connection-oriented communications is that nodes on a high-order layer are connected by a low-order layer network (link and nodes on a low-order layer). In this case, the nodes on the high-order layer are connected by a low-order layer connection established on the links and the nodes on the low-order layer, thus establishing a neighboring relationship between the nodes on the high-order layer. Typical examples of this type of network are a network wherein MPLS nodes (called LSRs (Label Switching Routers) and known also as label packet routers) connected by an optical wavelength network, and an MPLS network hierarchized by a label stack and the like. Thus, there are cases where the relationship between the high-order and the low-order is established between different layers and between layers of the same category.
If the network is configured by the MPLS network and the optical wavelength network, the optical wavelength network corresponds to the low-order layer network, while the MPLS network corresponds to the high-order layer network. The MPLS nodes are connected to each other via optical paths (OPs) corresponding to the links and optical switches (optical cross connects (OXCs)) corresponding to nodes on the low-order layer, and are further logically connected via connections (LSPs (Label Switched Paths)) established on the optical paths and the optical cross connects on the high-order layers. If the two LSRs are directly (without via other LSRs) connected by the high-order link, these LSRs are adjacent (neighboring) to each other.
If there are three or more pieces of LSRs, the LSP might be established in a state of extending across one or more LSRs. For example, if the MPLS network accommodates three LSRs functioning as an ingress (start) router, an intermediate (relay or forwarding) router and an egress (end) router, the ingress LSR and the egress LSR are connected by an LSP extending via the intermediate LSR. In this case, the intermediate LSR is bypassed due to a change in topology of the optical wavelength network and so on, and the ingress LSR and the egress LSR may be directly connected by the link. This is known as “shortcut (cut-through)”. When the “shortcut” is carried out, the adjacent relationship between the LSRs changes. The “shortcut” is performed in order to keep a QoS (Quality of Service) in the communications if a performance of, e.g., the intermediate LSR declines, or if a trouble occurs in the link to the intermediate LSR.
In the MPLS network, a packet having information set in its MPLS header (which may be called a labeled packet) is forwarded on the LSP (between the LSRs) according to the label value that is set the MPLS header. The label value is a unique value determined for (assigned to) a link between the neighboring LSRs and has validity only between the neighboring LSRs. Hence, if the adjacent relationship between the LSRs changes because of the shortcut routing, the label values used so far become invalid. Accordingly, if the shortcut occurs, a process of assigning a new label value is executed.
For instance, as shown in FIG. 13A, there is assumed a network wherein routers LSR-A, LSR-B, LSR-C and LSR-D are connected by the optical wavelength network including optical cross connects OXC-1, OXC-2 and OXC-3. Further, there is considered a case in which optical paths (high-order links) configuring the LSP extending between the LSR-A and the LSR-D are established respectively between the LSR-A and the LSR-B, between the LSR-B and the LSR-C and between the LSR-C and the LSR-D. Namely, the case to be considered is that the LSP extending from the LSR-A via the LSR-B and the LSR-C to the LSR-D is established.
In this case, the respective LSRs-A, B, C and D determine the label values used for forwarding the packet by utilizing a protocol such as LDP (Label Distribution Protocol) and others. At this time, for example, as the packet forwarding label values, a label value “L1” is determined for the link between the LSR-A and the LSR-B, a label value “L2” is determined for the link between the LSR-B and the LSR-C, and a label value “L3” is determined for the link between the LSR-C and the LSR-D. These label values assigned locally between the LSRs. When establishing the LSP between the LSR-A and the LSR-D, the label values “L1”, “L2” and “L3” are, after the protocol such as LDP has been exchanged between the LSRs-A, B, C and D, assigned to the links between these LSRs. Thereafter, each of the LSRs forwards the received packet to the neighboring LSR in accordance with the label value attached to this packet. For example, the LSR-C, when receiving the packet attached with the label value “L2” from the LSR-B, replaces the label value “L2” with the label value “L3” to be attached to the packet and forwards this labeled packet to the LSR-D.
Herein, as shown in FIG. 13B, there is considered a case where the OXC-1 links the optical path between the LSR-A and the LSR-B to the optical path between the LSR-B and the LSR-C with the result that the LSR-B is shortcut. In this case, the LCR-C receives the packet attached with the label value “L1” from the LSR-A instead of receiving the packet attached with the label value “L2” from the LSR-B. The LSR-C is not, however, learned about receiving the packet attached with the label value “L1”. Hence, the LSR-C is unable to specify the LSR (an output interface for the packet) to which the packet should be forwarded. As a result, the LSR-C is unable to forward the packet attached with the label value “L3” to the LSR-D.
The packet forwarding from the LSR-A to the LSR-D after the shortcut has been done in a way that avoids such a situation needs a scheme of reassigning valid label values to the links between the LSR-A and the LSR-C and between the LSR-C and the of reassigning a label value “L4” to the link between the LSR-A and the LSR-C and a label value “L5” to the link between the LSR-C and the LSR-D in accordance with the protocol such as LDP.
The following problems, however, arise when reassigning the above label values each time the shortcut is carried out.
A problem (A) is that a processing load on the protocol for assigning the labels on the high-order layer rises.
A problem (B) is that the communications are interrupted during a period till the label assignment on the high-order layer is completed after the shortcut on the low-order layer has been done.
Thus, according to the prior art, the shortcut must be performed in order to keep the QoS etc in the communication in some cases, and the shortcut routing has a problem of causing an increased cost technically and in time. In general, this problem happens when the route of the connection is changed in the connection-oriented communication network.